Prior Art
Processes for preparing blends of polyolefin resins and of polyolefin rubbers are well-known; for example, blends having improved characteristics with respect to simple mechanical mixing can be obtained, according to U.S. Pat. No. 3,758,643, from blends of polyolefin resins and of ethylenepropylene copolymeric elastomers (EPM) or of ethylenepropylene-diene terpolymer elastomers (EPDN) partially vulcanized with peroxides, sulfur, sulfur-based vulcanizing systems and associated activators and accelerators.
Vulcanization of ethylene-propylene-diene terpolymer elastomers with systems constituted by alkylphenolic resins which have methylolic groups adjacent to the phenolic groups and are activated by halogen donors is suggested by U.S. Pat. No. 3,287,440 and is said to provide considerable improvements in mechanical characteristics in U.S. Pat. No. 4,311,628, where it is used to vulcanize blends of polyolefin resins and EPDM elastomers.
Another form of activation of alkylphenolic resins or of phenolic compounds having methylolic groups adjacent to the phenolic group is constituted by carboxylic acids, according to what is described in IT 1203607.
The vulcanization processes by means of which the elastomeric component of the blend constituted by thermoplastic polyolefin resins and EPM and EPDM elastomers is vulcanized by resorting to the above mentioned vulcanizing systems may be of various kinds.
The EPM or EPDM elastomer can be preliminarily partially vulcanized, as described in U.S. Pat. No. 3,758,643, in a steam curing device, in roll-mills, in internal mixers and then mixed with the polyolefin thermoplastic resin at a temperature above the melting temperature of the polyolefin resin, or can be vulcanized completely, in the absence of thermoplastic polyolefins, according to conventional methods, subsequently reducing the product obtained to a powder and mixing it with the polyolefin resin according to what is described by U.S. Pat. No. 4,311,628.
However, the preferred process is dynamic vulcanization, by means of which the polyolefin resin, the polyolefin elastomer, the vulcanizing system and possibly the fillers, pigments, antioxidants and plasticizers are mixed the at vulcanization temperature until vulcanization is completed by using conventional equipment for rubber processing, such as cylinder mixers, Banbury, Brabender or mixing extruders such as Buss, Werner Pfeiderer and the like, as described in EP 107.635.
With the dynamic vulcanization process, all the ingredients, with the exception of the vulcanizing system, are mixed at a sufficiently high temperature and for the time required to obtain thorough mixing of all the components; finally, the vulcanizing system is added and mixing is continued at the temperature and for the time required to obtain the desired degree of vulcanization, as described in U.S. Pat. No. 4,130,535. This last document, in particular, describes a master batch which contains the polyolefin resin and the elastomer. The master batch is partially vulcanized with a reduced amount of vulcanizer, and the product obtained is dynamically reprocessed with an additional amount of vulcanizer.
In compounds in which the vulcanizing system also includes a vulcanizing system activator, the activator is added to the blend prior to the addition of the vulcanizer. In many cases a sort of preliminary master batch is prepared which contains the elastomer and most of the polyolefin resin and of the oil, reserving for a subsequent operation the dynamic vulcanization during which any further amounts of oil and/or polypropylene are added in addition to the vulcanizing system.
The product obtained with the process of dynamic vulcanization substantially consists of particles of vulcanized rubber dispersed in a matrix constituted by the thermoplastic polymer.
The mechanical and physical characteristics of the product obtained depend to a large extent on the dimensions and nature of the vulcanized particles of the elastomer and on their distribution within the thermoplastic matrix.
However, the conditions in which dynamic vulcanization occurs are themselves the cause of a series of drawbacks, including poor thermoplasticity and problems in the shaping of the finished item which arise especially when the ratio between polyolefin elastomer and polyolefin resin is relatively high and the presence of particles which are not perfectly dispersed or have a different hardness than the remaining material is often visible even to the naked eye.
The blend of the polyolefin resin and of the polyolefin elastomer in fact constitutes a heterogenous system, in which each one of the polymeric species acts as dispersant with respect to the other. Addition of the vulcanizing system is performed when the blend of the polymers is at such a temperature as to trigger vulcanization before the system is adequately dispersed in the reaction mass, so that particles of elastomer will find themselves in contact with concentrations of the crosslinking system which are much higher or lower than those required for the proper vulcanization, leading to scorching in some elastomeric particles and to undervulcanization in others.
In fact, considering for example a polyolefin resin based on polypropylene, in order to allow adequate mixing of the vulcanizing system it is obviously necessary to work above the melting temperature of the polypropylene, which melts at 155.degree.-165.degree. C., but in order to obtain the required fluidities it is more convenient to use temperatures not lower than 180.degree. C. and preferably close to or higher than 200.degree. C. At these temperatures, which can be obtained by friction or by heating, vulcanization occurs very quickly. With some vulcanization systems, for example, at 150.degree. C. the required vulcanization time is approximately 30 minutes, but for every increase of 7.degree.-10.degree. C. in temperature the vulcanization rate doubles.
For the same reasons, although some patents, such as for example U.S. Pat. No. 4,130,535, link the crosslinking rate to a given content of non-extractable material, in practice it is very difficult and onerous to control the degree of maximum crosslinking to be obtained. In fact, in order to evaluate this degree one must resort to solution tests in cyclohexane at room temperature or in xylene at boiling point, with rather significant margins of error, considering that olefin elastomers and particularly EPDMs often have, even prior to crosslinking, a certain percentage of gels which must be taken into account in tests conducted in the above mentioned solvents.
It is well-known that in conventional elastomer technology, the use of additives in solid powder form is prevailing whenever possible, and even liquid vuloanizers or vulcanizing systems are preferably absorbed or dispersed in powdery solid substances.
This is due to the fact that the high viscosity of almost all elastomers and the relatively low value of the compounding temperatures generally maintained makes it difficult to easily mix liquids with the elastomer.
However, although vulcanizing systems and their additives are produced as very fine powders and although they are then finely dispersed in the elastomeric, undervulcanization and overvulcanization points may form during vuloanization; the latter, in particular, show up as gels which are visible even to the naked eye in the finished product, especially if it is manufactured by extrusion.
Similar problems arise from the use of vulcanizers constituted by phenols or phenolic resins which have a methylolic group or groups adjacent to the phenolic group, and by phenols or halogenated phenolic resins which have a methylolic group or groups adjacent to the phenol.
These materials are available in the form of solids, flakes or coarse powder; however, their melting point, which is around 45.degree.-55.degree. C., and their considerable stickiness above room temperature make it difficult to rapidly and uniformly disperse the vulcanizer in the elastomer, especially in the conditions of dynamic vulcanization.
It is known to perform a preliminary mixing of these phenolic vulcanizers with polyolefins, subsequently converting the blend into granules. IT 1223262 instead describes the use of a commercially available product constituted by a master batch in powder form produced by the Schenectady company, which contains 50% of the phenolic resin commercially known as SP 1045 and 50% of barium sulphate.
Nevertheless, both are solid particles which must obviously be dispersed in the best possible manner in the elastomer or in blends which contain the elastomer while vulcanization begins simultaneously.
Due to the above, the characteristics of the equipment and more generally of the processes for processing elastomers entail the use of solid materials which are in powder form, or are or can be reduced to a solid powder form during processing. Nevertheless, the achievement of a more close and uniform contact of the vulcanizing system or of its main components with the elastomer to be vulcanized is highly desirable.